I've often wondered this too. I suspect that a family making use of WIC, medicaid / medicare and/or disability is commonly using more than $10k/person/year or gov't assistance, and would lose money going to UBI at that rate. I also wonder what could be done to avoid the UBI version of the two-income trap[0] - seems like UBI would naturally pressure entry level wages downward.

http://www.amazon.com/The-Two-Income-Trap-Middle-Class-Paren...